qa: retire kclient-specific layout test

`cephfs set_layout` was broken and is now deprecated
in favour of using xattrs for layout.  Retire the
kclient-specific test.

Fixes: #8773

Signed-off-by: John Spray <john.spray@redhat.com>
cephfs: pool safety & print deprecation warning

Print a message indicating to users that this tool
is now deprecated in favour of vxattrs.

Signed-off-by: John Spray <john.spray@redhat.com>
doc: add cephfs layout documentation

This clarifies how to deal with layouts in CephFS
using vxattrs.  We can point people here if they
ask what they should use instead of the deprecated
`cephfs set_layout`.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2107 from ceph/wip-set_layout

Wip set layout

Reviewed-by: Greg Farnum <greg@inktank.com>